Future interest rates and inflation cannot be reliably predicted! But IMO your proposed strategy is reasonable if you are fairly certain that you won't need the money before 5 years and think that you may be leaving it for the full 10 years.
Pure speculation on my part but I think a 13.35 net return over 5 years may well beat inflation over that period. The CPI in the year to March 2013 was +0.5% while the HICP was +0.6%. Also, the return on State Savings appears to be well insulated from DIRT increases which IMO are a certainty in coming budgets.
Personally, I have a mix of State Savings products, most of it is in the 3 year and 5.5 year with a small amount in the 4 year and 10 year. So I have a wide variety of maturity dates. However the interest rates for reinvestment are obviously lower now.